System and Process For Removal Of Organic Carboxylates From Mono Ethylene Glycol (MEG) Water Streams By Acidification and Vaporization Under Vacuum

ABSTRACT

A system and method for removing organic carboxylates from a mono ethylene glycol (“MEG”) stream includes a reaction vessel; means for cooling and diluting the MEG stream being routed to the reaction vessel; means for acidifying the cooled and diluted MEG stream during its residence time within the reaction vessel; and means for removing an acetic-rich overhead stream from the reaction vessel. The acidification of the cooled and diluted MEG stream occurs under a vacuum. The reaction vessel may be located downstream of a calcium removal vessel and receive a filtered bottom stream from that vessel, or it may be a single reaction vessel that cycles between a calcium removal mode and an acetate removal mode, with the pressure of the single vessel being greater during the calcium removal mode than during the acetate removal mode.

BACKGROUND OF THE INVENTION

This invention relates to systems and methods designed to treat monoethylene glycol (“MEG”) streams used in the oil and gas industry tocontrol hydrates formation in the production pipeline. Moreparticularly, the invention relates to systems and processes which allowfor removal of carboxylates from the MEG water stream of a MEGreclamation and regeneration package to reduce MEG losses.

In the oil and gas industry, dry (lean) MEG is injected into theproduction pipeline to control the formation of hydrates within theproduced stream. The MEG injection is part of a MEG loop of a gasproduction facility. The loop typically includes a reclamation andregeneration package to treat the wet (rich) MEG and reclaim as much MEGas possible for reinjection into the pipeline.

The formation waters and condensed waters, which arrive at the gasproduction facility along with the raw hydrocarbon products, containorganic acids. Because these organic acids are highly soluble inMEG-water mixtures, they tend to follow the MEG-water stream.Additionally, in order to protect production pipelines againstcorrosion, and to remove dissolved divalent cations from the MEG stream,the pH of the MEG-water in the pipeline is elevated by the addition ofbases such as sodium hydroxide. At elevated pH, the organic acids arepresent as a carboxylate salt (e.g., sodium acetate).

The low volatility of the carboxylate salts results in theiraccumulation in MEG process streams within the MEG loop. Thisaccumulation, in turn, results in increased viscosity and density,making the MEG streams more difficult to pump.

To control the carboxylate levels in the MEG loop, the carboxylate-richMEG is periodically discharged. However, this results in loss of MEGfrom the system and requires replacement to ensure the MEG inventory ofthe loop is maintained. Therefore, a need exists for systems andprocesses which control the carboxylate levels and reduce or eliminateMEG loss in the loop.

Carboxylate accumulation (either as the organic acid or as thecorresponding salt) is an issue for MEG reclamation and regenerationpackages due to the high solubility of these species in the water-MEGaqueous phase.

In order to minimize corrosion issues, the MEG Loop is operated atrelatively high pH whereby the carboxylic acids are presentpredominantly as the carboxylate salts which have low volatility and,thus, are not removed in the overheads (produced water) stream from theregenerators or reclaimers of the MEG Recovery Package. Their highsolubility in alkaline MEG solutions means that they do not precipitatewhen the pH is raised to remove the calcium, magnesium and otherdivalent cations.

Accumulation of acetates can lead to elevated density and viscosity inMEG streams which, in turn, lead to operational difficulties. Therefore,a need exists for a system and process to remove organic carboxylatesfrom the MEG water stream.

SUMMARY OF THE INVENTION

By employing a system and process made and practiced according to thisinvention, the problems discussed in the above background section areminimized because the acetate levels are controlled to a manageablelevel while MEG losses associated with the acetate removal process arekept to a minimum when compared to the alternative.

A system and process for removing organic carboxylates from a monoethylene glycol (“MEG”) stream includes a reaction vessel; means forcooling and diluting the MEG stream being routed to the reaction vessel;means for acidifying the cooled and diluted MEG stream during itsresidence time within the reaction vessel; and means for removing anacetic-rich overhead stream from the reaction vessel. The acidificationof the cooled and diluted MEG stream occurs under a vacuum.

The reaction vessel may be located downstream of a calcium removalvessel and receive a filtered bottom stream from that vessel, or it maybe a single reaction vessel that cycles between a calcium removal modeand an acetate removal mode, with the pressure of the single vesselbeing greater during the calcium removal mode than during the acetateremoval mode.

Preferably, the cooling and diluting means results in the incoming MEGstream to be 50 wt % MEG at a temperature in a range of 80° to 100° C.In the acetate removal mode, the pressure is sub-atmospheric, preferablyin a range of 0.1 to 0.3 bar. The acidifying means, which may behydrochloric acid, results in the cooled and diluted MEG stream duringits residence time within the reaction vessel to have a pH in a range of3.5 to 5.5.

Objects of the invention include providing a system and process which:(1) can be retrofitted into existing MEG loops; (2) controls and reducesthe amount of acetates in the MEG water stream; (3) extends the lengthof the production run; (4) reduces MEG loss; and (5) increases MEGrecovery for re-use.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a preferred embodiment of a system and process practicedaccording to this invention. A single reaction vessel is used for theremoval of calcium and carboxylates from a mono ethylene (MEG) waterstream. The reaction vessel swings or cycles between those two removalmodes. The acetate-rich overhead stream is routed directly to aknock-out drum.

FIG. 1B is another preferred embodiment of a system and processpracticed according to this invention. A single reaction vessel is usedfor the removal of calcium and carboxylates from a mono ethylene (MEG)water stream. Similar to FIG. 1A, the vessel cycles between those tworemoval modes. The acetate-rich overhead stream is routed directly to adistillation column where it is mixed with a MEG-water overhead streamfrom a reclaimer.

FIG. 2 is yet another preferred embodiment of a system and processpracticed according to this invention. One reaction vessel is used forthe removal of calcium from the MEG-water stream and another reactionvessel is used for the removal of carboxylates. The high pH streamgenerated in the calcium removal vessel is filtered to remove solids andthen treated with hydrochloric acid in the acetate removal vesseloperated at sub-atmospheric pressure.

FIGS. 3 to 7 show simulated results of a system and process madeaccording to FIGS. 1A, 1B and 2 for acetate removal from a modelsolution where solution pH was reduced to 4.5 prior to raisingtemperature and reducing pressure.

FIG. 3 is a graph of acetate removal as a function of temperature andpressure.

FIG. 4 is a graph of MEG losses in the reaction vessel overheads as afunction of temperature and pressure.

FIG. 5 is a graph of acetate removal against MEG loss for threetemperatures (80, 90 and 100° C.).

FIG. 6 is a graph of calculated MEG loss per kg of acetate rejected as afunction of operating pressure.

FIG. 7 is a graph of calculated acetate removed from the reaction vesselas a function of initial pH (3.5, 4.5, 5.5), temperature (60-100° C.),and operating pressure (0.1-0.3 bara).

FIG. 8 is schematic of an apparatus used to test preferred embodimentsof the system and process of this invention.

ELEMENTS AND ELEMENT NUMBERING USED IN THE DRAWING FIGURES

-   10 System-   11 Reaction vessel-   13 Incoming carboxylates-rich MEG stream-   15 Water stream-   17 Cooled and diluted MEG water stream-   19 Precipitating means-   21 Acidifying means-   23 Acetate-rich overhead stream-   25 Condensed acetate-rich overhead stream-   27 Calcium carbonates bottoms stream-   29 Filtered calcium carbonates bottoms stream

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A system and process made and practiced according to this inventionallows a target salt (in this case acetate) to accumulate in a reactionvessel and then removes the acetate from a concentrated liquor withinthe reaction vessel, thereby keeping vessel site and inventory to aminimum.

Referring to the drawings and first to FIGS. 1A and B, a system 10 forremoving organic carboxylates like acetates from a MEG water streamincludes a reaction vessel 11 as part of the MEG loop of a MEGreclamation and regeneration package. The MEG reclamation unit (notshown) is typically operated at a temperature in a range of about120°-140° C. and at sub-atmospheric pressure (about 0.1-0.3 bara). Therecycle loop within the reclamation unit is operated at elevatedpressure (about 4 bar). The reaction vessel 11 is closely coupled to therecycle loop of the package.

The incoming carboxylate-rich MEG stream 13 is typically at 80-90 wt %MEG and high ph (>9.5). The incoming MEG stream 13 is cooled and dilutedwith water 15 to yield a cooled and diluted MEG water stream 17 whichenters and resides within reaction vessel 11 as a MEG water mixture. Thecooled and diluted MEG stream 17 is preferably at a temperature of about80-100° C. and 50 wt % MEG.

The reaction vessel 11 can be switched between a calcium removal mode(high pH, atmospheric pressure) and an acetate removal mode (low pH,sub-ambient pressure). The frequency of calcium removal cycles andacetate removal cycles can be varied to control the levels of calciumand organic acids in the MEG loop depending on the composition of theMEG feed entering the MEG regeneration package.

When reaction vessel 11 is in a calcium removal mode or cycle, thevessel 11 operates at atmospheric pressure and removes calcium and otherdivalent cations from the incoming MEG water stream by elevating pH.Precipitating means 19 such as sodium or potassium carbonate or sodiumor potassium hydroxide are introduced to the reaction vessel 11. Saltsresiding in the MEG water mixture—such as calcium chloride and,commonly, lesser amounts of other divalent salts like magnesium, bariumand strontium chlorides—react with the precipitant agent and precipitateout of the MEG water mixture as solid crystals. The solid crystals areremoved as a bottom stream 27.

When reaction vessel 11 is in an acetate removal mode, the vessel 11operates under a vacuum (preferably in a range of about 0.1-0.3 bar) andremoves carboxylates by lowering pH (preferably in a range of about3.5-5.5). The stream 17 is acidified within the reaction vessel 11 usingacidifying means 21 such as hydrochloric acid (e.g., 30 w % HCl inwater) to achieve a pH in a range of about 3.5-5.5. The pressure inreaction vessel 11 is then reduced to 0.1-0.3 bar and acetates areevolved along with water, some carbon dioxide and some MEG. Means wellknown in the art are employed to remove the acetic-rich overhead stream23 from the vessel 11.

The composition of the overhead stream 23 from the reaction vessel 11 isprimarily a function of temperature and pressure (see FIGS. 3 & 4).Ideally, the overhead stream 23 contains the maximum quantity of aceticacid and a minimum quantity of MEG. The results reported below in FIGS.3 & 4 can be applied to determine an optimum initial pH, temperature andpressure regime whereby the maximum yield of acetic acid is combinedwith a reduced yield of MEG in the stream 23.

In the system of FIG. 1A, the overhead stream 23 is condensed and thecondensed stream 25 (low pH<3) may be routed to water treatmentequipment or neutralized in a knock-out drum. In the system of FIG. 1B,the overhead stream 23 is condensed and the condense stream 25 is routeddirectly to the distillation column where it is mixed with a MEG-wateroverhead stream from the reclaimer. The acetates partition between theaqueous phase (produced water) and the lean MEG phase. Routineexperimentation can be used to determine the partitioning of acetic acidbetween water and lean MEG to determine the preferred routing of aceticacid-MEG-water stream from the acetate removal process.

Referring now to FIG. 2, separate vessels 31, 33 are used for calciumremoval and carboxylates removal, respectively. During simultaneousproduction of calcium and carboxylic acids the calcium (and otherdivalent cations) are precipitated in vessel 31 at approximately 80° C.and 1.0 bar by raising the pH using precipitating means 19 and filteringthe resulting calcium carbonates stream 27. The centrate/filtrate steam29 is acidified using acidifying means 21 at 80-100° C. and atsub-atmospheric pressure (about 0.1 to 0.3 bar) in vessel 33 to removethe organic acids and water as an overhead stream 23. The calcium-freeand organic acid-free MEG can be returned to the production process.

However, the same calcium removal process as that described for vessel11 (see FIGS. 1A and 1B) is employed within vessel 31, as is the sameacetate removal process for vessel 33.

Simulated Results

Simulated results were obtained employing OLI STREAM ANALYZER™ software(OLI Systems, Inc., Cedar Knolls, N.J. A model feed representing a highpH, 50% MEG solution with 3 wt % dissolved sodium acetate and excesssodium hydroxide and sodium bicarbonate was reacted with hydrochloricacid (as HCl) to reduce the pH to 3.5-5.5. The temperature of thesolution and the reaction pressure were adjusted and the composition ofthe predicted overhead stream was calculated. The acetate content of thereaction mixture was fixed at 30 kg sodium acetate.

FIG. 3 shows acetate removed as a function of temperature and pressure.The highest levels of acetate are removed at elevated temperature andlow pressure. FIG. 4 shows that MEG losses are highest at elevatedtemperature and reduced pressure. Therefore, an optimumtemperature/pressure condition is required to maximize acetate removalwhile reducing or minimizing MEG losses to acceptable levels.

FIG. 5 plots acetate removal against MEG loss for three temperatures(80, 90 and 100° C.). Lower temperatures and low pressure are preferredto maximize acetate removal. FIG. 6 plots MEG loss as a function ofacetate rejected from the MEG Loop. Using simple blowdown, 950 kg of MEGwill be ejected with every 30 kg of sodium acetate (44 kg MEG per kg ofacetate).

FIG. 7 plots acetate removal as a function of temperature and pressurefor three initial pH conditions: 3.5, 4.5, 5.5). Acetate removalefficiency is increased as the starting solution pH is reduced. For astarting pH of 3.5, 89% of the acetate in the reaction vessel is removedin the overhead stream at 100° C. and 0.1 bara compared to 48.3% for aninitial solution pH of 4.5 and 8.8% for an initial solution pH of 5.5.

Using the acidification/vaporization scheme practiced according to thisinvention can significantly reduce this MEG loss. At 80° C. and 0.15bar, the simulation software predicts 73.8 kg of MEG in the overheadstream along with 8.75 kg of acetic acid (8.58 kg MEG per kg acetate).At 80° C. and 0.15 bar only 40% of the acetate is removed per batch withthe acetate remaining in the liquid phase being routed back to thereclaimer for re-processing.

Experimental Results

The apparatus used in the test is shown in FIG. 8. A double skinned 5Lglass reactor vessel 101—fitted with a pH probe 102 (a Hamilton PolilytePlus ARC 425), a dissolved oxygen probe 103 (Hamilton Oxygold G ARC425), a RedOx probe 104 (Hamilton Polilyte Plus ORP ARC 425), aelectrical conductivity probe 105 (Hamilton Conducell 4USF ARC PG425),and a stirrer 106—was heated by means of a hot oil circulator bath andheater 107. The reactor vessel 101 was connected to a condenser 108 anda distillate collection vessel 109. The reactor vessel 101, condenser108, and distillate collection vessel 109 were evacuated using a vacuumpump 110, a pressure control valve 111 and a pressure transducer 112. Anoxygen-free nitrogen supply 113 was connected to the reactor vessel 101to provide gas blanketing function.

A MEG-water-acetic acid solution was prepared in the reactor vessel 101by adding 93 g of acetic acid (99-100% ex Sigma-Aldrich) to a mixture ofmonoethyleneglycol (1,737 g, Uninhibited CoolFlow MEG ex HydraTechnologies Limited, Fforestfach, Swansea SA5 4AJ, UK) and de-ionisedwater (1,710 g). The pH of this solution was measured as 2.61 at 23° C.The acetate content of the solution was calculated as 2.58 wt %.

The pH of the test mixture was elevated to 10.2 by addition of 100 g ofanhydrous sodium carbonate (ex Sigma-Aldrich) and 1 g of sodiumhydroxide pellets (ex Sigma-Aldrich). At this high pH, the acetic acidis converted to sodium acetate which is non-volatile and which would notbe removed from the reaction vessel by elevating the temperature andreducing the operating pressure.

In conventional MEG loops the organic acids in the formation water andcondensed water are usually present as the sodium salt:

2CH₃CO₂H(aq)+Na₂CO₃(aq)→2CH₃CO₂Na+CO₂(aq)+H₂O

CH₃CO₂H(aq)+NaOH(aq)→CH₃CO₂Na+H₂O

In order to effectively remove the dissolved acetate the pH of thesolution was reduced from 10.2 to 3.5 by addition of 70 g of 37 wt %hydrochloric acid solution (Sigma-Aldrich). The pressure in the reactorvessel 101 was reduced to 0.15 barA and the temperature was raised to80° C. The reactor vessel 101 was held at 0.15 barA/80° C. forapproximately 3.3 hours.

The reactor vessel 101 was allowed to cool and the residue in the vessel101 and the distillate collected in the MEG-water collection vessel 109were weighed and analyzed. The results are shown in Table 1 below.

Table 1 shows that the MEG level in the reactor vessel 101 rises from53.6 wt % to 93.7 wt % as the water component is removed in preferenceto the less volatile MEG at low pressure and elevated temperatures.Table 1 also shows that the acetate component in the reactor vessel 101is also removed in preference to the MEG component and that the acetatecontent of the distillate (predominantly water) is higher (29,093 mg/L)than in the original reactor mixture (measured at 20,994 mg/L,calculated from starting composition at 25,396 mg/L).

TABLE 1 Analysis of Reactor and Distillate Vessel Contents MEG EstimatedAcetate Acetate Inven- by GC Density by IC Calculated tory [Note 1][Note 2] [Note 3] [Note 4] (g) (wt %) (g/L) (mg/L) (g) Reactor @ t = 03,781 53.6 1,065 20,964 75.49 Reactor @ t = 2,079 93.7 1,109 17,32632.75 3.33 hrs Distillate @ t = 1,580 6.7 1,006 29,093 45.69 3.33 hrs[Note 1]: MEG content measured by Gas Chromatography [Note 2]: Densityestimated as a function of MEG:water:dissolved salt at 20° C. [Note 3]:Acetate content measured using Ion Chromatography [Note 4]: Acetate(g) =Acetate (mg/L) × Inventory (g)/Density (g/L)

Based on a total final acetate measurement of 78.44 g (32.75 g remainingin the reactor vessel 101 plus 45.69 g collected in the distillate), itis calculated that 58.3% in the acetate present as sodium acetate in thereactor vessel at pH=10.2 was removed as acetic acid by reducing pH to3.5 then reducing the pressure to 0.15-0.17 barA and raising thetemperature to 80° C.

Experimental results and predicted values from the OLI STREAM ANALYZER™are shown in Table 2, below. The OLI model predicts 70% removal ofacetate at 80oC/1.5 barA.

TABLE 2 Experimental acetate removal compared with simulated results pHTEMP PRESSURE ACETATE — deg C. barA REMOVED % OLI Prediction 3.50 800.15 70.0 Experimental 3.34-3.78 66-80 0.14-0.17 58.3

Summary

The above preferred embodiments of a system and method made and practiceaccording to this invention are not all possible embodiments. The claimslisted below define the scope of the invention, including equivalents tothe elements listed.

What is claimed:
 1. A system for removal of organic carboxylates from amono ethylene glycol (“MEG”) stream, the system comprising: a reactionvessel; means for cooling and diluting the MEG stream being routed tothe reaction vessel; means for acidifying the cooled and diluted MEGstream during its residence time within the reaction vessel; and meansfor removing an acetic-rich overhead stream from the reaction vessel;wherein the acidification of the cooled and diluted MEG stream occursunder a vacuum.
 2. A system according to claim 1 wherein the reactionvessel includes an acetate removal mode and a calcium removal mode, thereaction vessel including means for alternating between the abovementioned modes, the pressure of the reaction vessel being greaterduring the calcium removal mode than during the acetate removal mode. 3.A system according to claim 1 wherein the reaction vessel is locateddownstream of a calcium removal vessel within a MEG recycle loop andreceives a bottom stream from the calcium removal vessel.
 4. A systemaccording to claim 1 further comprising means for raising the alkalinityof the cooled and diluted MEG stream during its residence time withinthe treatment vessel.
 5. A system according to claim 4 wherein theraising alkalinity means is selected from the group consisting of sodiumcarbonate, potassium carbonate, sodium hydroxide, and potassiumhydroxide.
 6. A system according to claim 1 further comprising means forremoving a calcium-rich bottom stream from the reaction vessel.
 7. Asystem according to claim 1 wherein the reaction vessel is at a pressurein a range of 0.1 to 0.3 bar.
 8. A system according to claim 1 whereinthe acidifying means results in the cooled and diluted MEG stream duringits residence time within the reaction vessel having a pH in a range of3.5 to 5.5.
 9. A system according to claim 1 wherein the acidifyingmeans is hydrochloric acid.
 10. A system according to claim 1 whereinthe cooled and diluted MEG stream is below 80 wt % MEG.
 11. A systemaccording to claim 10 wherein the cooled and diluted MEG stream is about50 wt % MEG
 12. A system according to claim 1 wherein the cooled anddiluted MEG stream is at a temperature in a range of 80° to 100° C.